Eight catastrophic floods in 11 days: What’s behind intense rainfall around the world?
by Denise Chow  /  Sept. 12, 2023

“The catastrophic flooding in Libya that is feared to have left as many as 10,000 people dead is just the latest in a string of intense rain events to hammer various parts of the globe over the past two weeks. In the first 11 days of September, eight devastating flooding events have unfolded on four continents. Before Mediterranean storm Daniel sent floodwaters surging through eastern Libya, severe rain inundated parts of central Greece, northwestern Turkey, southern Brazil, central and coastal Spain, southern China, Hong Kong and the southwestern U.S.

Seeing that many unrelated extreme weather events around the world in such a short time is unusual, said Andrew Hoell, a research meteorologist at the National Oceanic and Atmospheric Administration’s Physical Sciences Laboratory. “Sometimes we have a clustering of these events, whether it’s in a given country, given hemisphere or globally,” he said. “And it seems like right now, globally, this is prime time for a number of flooding events.” As with many other forms of extreme weather, scientists say climate change is most likely having an impact on rainfall and flooding, but understanding precisely what that relationship is can be tricky. In general, studies have shown that global warming is intensifying the planet’s water cycle. Warmer temperatures increase evaporation, which means a warmer atmosphere can hold more moisture. As a result, when storms can unleash more intense precipitation and thus cause severe flooding.

Researchers have observed those changes over time as the world warms. Since 1901, global precipitation has increased at an average rate of 0.04 inches per decade, according to the Environmental Protection Agency. However, a number of factors can influence flooding events and their severity, and teasing out the fingerprints of climate change when they all interact can be challenging, Hoell said. “From a 1,000-foot view, it’s definitely true that if you have higher temperatures, you have more water vapor, and therefore you can have more precipitation fall from the sky,” he said. “But when you look at a specific event and the specific set of physical processes relevant to that event, it then becomes difficult to attribute every single process in that causal chain.” For one thing, the types of extreme weather that caused each of the eight catastrophic flooding events this month had different origins.

It was a Mediterranean storm named Daniel that dumped heavy rain over central Greece and Libya. Typhoon Haikui and its remnants lashed Hong Kong and southern China with record rain, waterlogging urban and rural areas, destroying roads and causing more than 100 landslides. Torrential downpours caused flash flooding in central and coastal regions of Spain, northwest Turkey and thousands of miles away in the Brazilian state of Rio Grande do Sul. And fast-moving thunderstorms over southern Nevada this month caused flash flooding across the region, swamping the Las Vegas Strip and stranding more than 70,000 people at the Burning Man festival in Black Rock Desert.

With certain types of extreme flooding events, such as those associated with Mediterranean cyclones like Daniel, there simply isn’t enough data to observe shifts over time. “We really don’t have a long enough sample or record to be able to detect a change, because they’re not really that common of an occurrence,” Hoell said. In other cases, local factors, such as how wet or dry the ground is or an area’s basic topography, can have enormous influences on how floods develop — and their consequences. Beyond loss of life and property, floods increase the risks of exposing people to waterborne pathogens, which have important implications for outbreaks of deadly disease. Hoell said the number of devastating floods this month is distressing, but he said he’s especially concerned about the situation unfolding in Libya. “If you look at the damage and the amount of people who have lost their lives,” he said, “it just blows your mind.”

Deadly flooding hit several countries at once. Scientists say this will only be more common
by Isabella O’Malley, Brittany Peterson and Drew Costely /  July 17, 2023

“Extreme rainfall accompanied by deadly flooding hit the United States and several other countries over the weekend and last week. There were several dozen fatalities in central and southern regions of South Korea, including the Chongju region where an underpass flooded and drowned motorists who became trapped in their submerged vehicles. In the U.S., flooding claimed five lives in Upper Makefield Township, Pennsylvania, where a search is ongoing for two missing children. Flooding also struck parts of New Hampshire, Massachusetts, Connecticut, New York and New Jersey this past weekend. A state of emergency was declared in New Jersey by Gov. Phil Murphy following significant damage from flooding and landslides.

This follows last week’s relentless flooding in India, Japan, China, Turkey and the U.S. Although the destructive floods are occurring in different parts of the world, atmospheric scientists say they have this in common: With climate change, storms are forming in a warmer atmosphere, making extreme rainfall a more frequent reality now. The additional warming that scientists predict is coming will only make it worse. That’s because a warmer atmosphere holds more moisture, which results in storms dumping more precipitation that can have deadly outcomes. Pollutants, especially carbon dioxide and methane, are heating up the atmosphere. Instead of allowing heat to radiate away from Earth into space, they hold onto it.

While climate change is not the cause of storms unleashing the rainfall, these storms are forming in an atmosphere that is becoming warmer and wetter. “Sixty-eight degrees Fahrenheit can hold twice as much water as 50 degrees Fahrenheit,” said Rodney Wynn, a meteorologist at the National Weather Service in Tampa Bay. “Warm air expands and cool air contracts. You can think of it as a balloon—when it’s heated the volume is going to get larger, so therefore it can hold more moisture.” For every 1 degree Celsius (1.8 degrees Fahrenheit) that the atmosphere warms, it holds approximately 7% more moisture.

According to NASA, the average global temperature has increased by at least 1.1 degrees Celsius (1.9 degrees Fahrenheit) since 1880. “When a thunderstorm develops, water vapor gets condensed into rain droplets and falls back down to the surface. So as these storms form in warmer environments that have more moisture in them, the rainfall increases,” explained Brian Soden, professor of atmospheric sciences at the University of Miami. Along Turkey’s mountainous and scenic Black Sea coast, heavy rains swelled rivers and damaged cities with flooding and landslides.

At least 15 people were killed by flooding in another mountainous region, in southwestern China. “As the climate gets warmer we expect intense rain events to become more common, it’s a very robust prediction of climate models,” Soden added. “It’s not surprising to see these events happening, it’s what models have been predicting ever since day one.” Gavin Schmidt, climatologist and director of the NASA Goddard Institute for Space Studies, said the regions being hit hardest by climate change are not the ones that emit the largest amount of planet-warming pollutants. “The bulk of the emissions have come from the industrial Western nations and the bulk of the impacts are happening in places that don’t have good infrastructure, that are less prepared for weather extremes and have no real ways to manage this,” said Schmidt.

Deadly flooding hit several countries at once. Scientists say this will only be more common
“People wade through a street due to a heavy rain in Kurume, Japan on July 10, 2023”

In last week’s flooding, schools in New Delhi were forced to close on July 10 after heavy monsoon rains battered the Indian capital, with landslides and flash floods killing at least 15 people. Farther north, the overflowing Beas River swept vehicles downstream as it flooded neighborhoods. In Japan, torrential rain pounded the southwest, causing floods and mudslides that left two people dead and at least six others missing. Local TV showed damaged houses in Fukuoka prefecture and muddy water from the swollen Yamakuni River appearing to threaten a bridge in the town of Yabakei. In Ulster County, in New York’s Hudson Valley and in Vermont, some said the flooding is the worst they’ve seen since Hurricane Irene’s devastation in 2011.”

This summer’s extreme weather is a sign of things to come
by Kieran Mulvaney  /  September 8, 2023

“Two-thirds of the way through the year, 2023 seems destined to be remembered as the year that extreme weather events left the Northern Hemisphere reeling. The summer months brought a litany of extreme weather events: Wildfires across CanadaFlooding in Vermont. An “unusually early and aggressive start” to the Atlantic hurricane season. A devastating wildfire on the Hawaiian island of Maui, fueled by heat and cyclonic winds, that all but destroyed a historic town..

According to the European Union’s Copernicus Climate Change Service, the three-month period from June to August was the warmest northern hemisphere summer ever recorded, 1.2° F above the 1991-2020 average and 0.5 degrees higher than the previous record. And a new analysis by the nonprofit organization Climate Central has drawn a direct line between those temperatures and climate change, arguing that nearly half of the global population—3.9 billion people—experienced 30 or more days between June and August with temperatures made at least three times more likely by climate change.

“Virtually no one on Earth escaped the influence of global warming during the past three months,” Andrew Pershing, Climate Central’s vice president for science, said in a press release. “In every country we could analyze, including the southern hemisphere where this is the coolest time of year, we saw temperatures that would be difficult–and in some cases nearly impossible–without human-caused climate change. Carbon pollution is clearly responsible for this season’s record-setting heat.”

Connecting the dots between climate change and this summer’s extreme weather is complicated, but some scientists are feeling increasingly confident in their ability to do so. Speaking at a press briefing to announce the Climate Central findings, Friederike Otto, a physicist at Imperial College London said that the intensity of wildfires in Quebec that blanketed large areas of the U.S. in smoke “was about 50 percent more intense because of human-induced climate change,” and that the weather conditions that allowed the fires to flourish were “made at least twice as likely” by a warming climate. But Otto, a co-founder of the World Weather Attribution initiative, acknowledged that, because of the multiple components involved in driving extreme weather events, “quantification is quite different depending on different methods and different models.”

For that reason, some scientists prefer to speak in more general terms about how climate change has driven the year’s extreme weather. “We can say, with a great deal of confidence, that we would not be seeing the unprecedented weather extremes we saw play out this past summer in the absence of human-caused warming from fossil fuel-generated carbon pollution,” says Michael Mann, director of the Penn Center of Science, Sustainability and the Media, in an email. But he argues that analysis that attempts to attribute specific events to climate change can sometimes miss the bigger picture of how warming affects broader, longer-term weather patterns, such as by distorting the jet stream, the band of westerly winds that circles the northern hemisphere. A number of researchers are concerned that a warming Arctic is weakening the jet stream, allowing hot air from the tropics to flow north and prolonging heatwaves, fueling wildfires, and melting the Arctic.

For that reason, explained Mann, whose latest book on climate change is publishing later this month, “I often prefer to talk about how climate change is impacting the basic processes relevant to all of these events, which includes not only the obvious fact that heat waves are more frequent and intense on a hotter planet, but the fact that hotter continents in the summer lose more moisture through evaporation.” What might seem like just a heat wave can set off a cascade of disasters, Mann notes. Worse droughts lead to more damaging wildfires. And a warmer atmosphere can hold more water vapor, so when rain does arrive it is often heavier and more likely to lead to flooding.

Adds Stephen MacAvoy of the Department of Environmental Science at American University, the key to understanding the future of weather is to assume an increase in the “extreme and erratic.” “Your current conditions won’t necessarily be exacerbated,” he explains. “If it’s dry, it doesn’t necessarily get drier. And if it’s wet, it won’t necessarily get wetter. But there’s a good chance it’s going to be odd, frankly. I mean, Texas didn’t expect to get frozen in 2021.” Where climate researchers are in accord, however, is that this past summer will not prove anomalous. We may not experience similarly widespread extreme conditions next year or the year after, but we will experience them again and again. “In terms of extreme weather:—well, we have to get used to that,” says MacAvoy. But he takes umbrage at the notion that 2023 presages a “new normal.”

“I have issues with that phrasing,” he explains. “Because normal implies stable, and we’re not in a new normal, because everything is still changing, and it’s going to continue to change.” Mann agrees. “I’ve sometimes favored the term ‘a new abnormal,’ but the basic problem with this framing is that it makes it sounds like we’ve arrived in some new state of the climate and just have to figure out how to adapt to it,” he explains. “But it’s much worse than that. As we continue to warm the planet, all of this gets worse and worse.”

Melting Ice Sheets Could Worsen Extreme Weather
by Chelsea HarveyE&E News  /  February 7, 2019

“The world’s melting ice sheets may have consequences far beyond global sea-level rise, scientists say. They may also change the ocean’s circulation, alter global temperature patterns and even drive extreme weather events around the world. Under a business-as-usual climate scenario, with warming likely exceeding 4 degrees Celsius by the end of the century, a major ocean current could slow down. Some parts of the world may cool off, while others could warm up even faster. Heat waves and other severe weather events might happen more frequently. And changes in the Southern Ocean may drive more warm water to the edge of Antarctica—causing the ice sheet to melt at even faster rates.

These findings, published yesterday in the journal Nature, join a growing body of research suggesting that melting ice may have a more complicated influence on the global climate system than previously thought. “It’s really just underpinning the fact that we are probably messing with something that we don’t fully understand,” said lead study author Nicholas Golledge, an Antarctic ice sheet expert at Victoria University of Wellington in New Zealand. Climate models don’t typically account for fresh water pouring into the ocean from melting Greenland and Antarctica. As a result, those consequences are missing from many simulations of the warming planet. The reason is in part because it’s difficult to accurately simulate ice sheet melting in global climate models, which take a broad look at the world as a whole. The Greenland and Antarctic ice sheets are complex enough that they generally require their own separate, high-resolution models. And until about 10 years ago, many scientists assumed that the melting ice sheets had little effect on the climate system as a whole, beyond their contributions to sea-level rise, Golledge noted.

In recent years, though, researchers have begun to suggest that the icy fresh water might be disrupting some major climate processes as it flows into the ocean, with the potential for dramatic effects worldwide. To address the modeling problem, the authors of the new study conducted a series of simulations. First, they used updated satellite observations of Greenland and Antarctica to simulate the melting ice sheets under future climate change. Then, they added those estimates of future ice melt to a global climate model simulation and compared its response with simulations with no meltwater influx included. Meltwater from Greenland, they found, may substantially affect the flow of a huge ocean current known as the Atlantic meridional overturning circulation, or AMOC. Akin to a gigantic conveyor belt, the AMOC helps ferry warm water from the tropics to the Arctic. Its important role in distributing heat around the world also gives it a major influence on global weather patterns—the AMOC’s warm-water flow is largely credited with the mild climate in places like Western Europe. As fresh water pours into the ocean from the melting Greenland ice sheet, it may cause this current to slow down. Some scientists have suggested that this process may have already begun.

As the transport of heat becomes more sluggish, climate patterns throughout the North Atlantic may begin to change. The new study suggests that some regions, including the Greenland continent, may actually experience a cooling effect. Other places, like the eastern Greenland sea on up to the North Pole, may warm up. Meanwhile, melting ice from Antarctica may upset the structure of the Southern Ocean, the research suggests. Because fresh water is less dense than salt water, large influxes of meltwater could fail to mix in with the rest of the ocean and instead form a layer that rests on the surface of the water. This cold sheet of liquid traps heat beneath it and causes deeper layers to warm up. Deepwater currents in the Southern Ocean carry this warm water to the edge of the Antarctic continent, where it bubbles up and laps against the edge of the ice.

The warmer the water, the more melting it’s likely to cause. Recent research suggests that warm ocean currents are already increasing melt rates at glaciers in some parts of the ice sheet, particularly West Antarctica. If even more warm water is able to reach the ice in the future, the melting could speed up. The simulations also suggest that the climate system overall may become more variable, or prone to fluctuations and disturbances like extreme weather events. Under a business-as-usual climate scenario, year-to-year variability may increase by as much as 50 percent. The research indicates that “polar ice sheets will have a crucial role in Earth’s climate in the future,” beyond what the models currently reflect, writes Helene Seroussi, an ice sheet expert with NASA’s Jet Propulsion Laboratory, in a comment on the new research.

These findings don’t come as a complete shock. Other recent research suggested that the melting ice sheets may have some far-reaching climate consequences. Multiple studies have now indicated that continued Greenland melting may slow the flow of the AMOC, disrupting climate and weather patterns throughout the North Atlantic. And one 2018 paper in Nature suggested that Antarctic melting under a business-as-usual warming scenario may slightly slow the rate of global warming. It could delay the approach of the 1.5 and 2 C temperature targets outlined under the Paris climate agreement by a decade, the research suggests. That study assumed a fairly extreme amount of ice melt by the end of the century, enough to raise global sea levels by about 3 feet. Some research has suggested that Antarctic glaciers may destabilize in a specific way under high levels of future warming—as ice shelves at the ocean’s edge melt and break apart, entire ice cliffs could collapse and unleash a massive outpouring of ice into the ocean. Three feet of sea-level rise might be expected by 2100 if this process were to actually occur.  Yet new research, also published yesterday in Nature, has challenged this idea, suggesting that this kind of ice cliff instability may not be so likely after all. That means future sea-level rise may be less extreme.

All of these findings underscore the fact that many components of the climate system—particularly those related to melting ice sheets—are still unfolding for climate scientists. Some aspects of future climate change may be more severe than models currently project, as this week’s new research points out. And some worst-case scenarios may not be as likely as they once seemed. To parse it all out, it’s critical to account for all the factors that might affect the progression of climate change when making predictions about the future, Golledge says. In this case, his new research points to the melting ice sheets as key participants in the global climate system, whose influence may have been previously overlooked. “I guess we’re just holding up a flag here and saying this is something we really need to look into,” Golledge said. “And we need to do it as well as we can with as many models as we can and figure out if actually we’re underestimating the consequences.”



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